In the prior art toroidal current transformers are well established as a method of measuring alternating currents. Referring now to FIG. 1, these devices typically consist of a toroidal ferromagnetic core and a coil. The coil, consisting of insulated copper wire, is wound around the core, usually in a way that conforms to the toroidal shape of the core. Alternating current in a conductor which passes through the center of the core creates a changing magnetic field which in turn induces a voltage at the coil terminals. This voltage provides a measure of the current in the conductor. If high accuracy is required or if the frequency is not constant, this voltage can be integrated by an analog integrator to give a more accurate measure of the current.
In theory if the coil and the core are toroidal and perfectly uniform and if the coil has an infinite number of turns, then the output voltage is unaffected by the position of the conductor passing through the center of the core. In other words the conductor does not need to be accurately centered to provide an accurate current measurement. In addition, the output voltage is unaffected by external fields produced by nearby currents which do not flow through the inside of the core. In practice, as long as the core is uniform and has a high magnetic permeability, the coil does not need to be uniform and does not need to have very many turns. Nevertheless, it may be advantageous to have a large number of turns in order to produce a large signal.
There are a number of drawbacks to this kind of current sensor. The presence of the ferromagnetic core will change the impedance of the AC conductor. Also, ferromagnetic cores can saturate and in the process produce non-linear signals. To prevent this, an electrical load or burden is often connected to the output terminals.
While such a sensor may be suitable for permanent installations, the need to string the conductor through the core can cause difficulties when retrofitting is required. Some current transformers have a split core to allow for easy installation. The problem with this is that the overall magnetic permeability of the core, and therefore the output signal, is affected by small variations in the gaps where the two halves of the split core meet. This variation can be greatly reduced by making the gaps large, but the consequences of this is that the overall magnetic permeability of the core is greatly reduced and many more turns are then required in the coil to produce the same signal output. Also, if the magnetic permeability of the core is lower, then the uniformity of the coil windings becomes more important.
Coil winding adds significantly to the cost of such a sensor. This cost is higher if more turns are required, and can be especially high if uniform winding is necessary. The winding of toroidal coils is especially expensive, particularly if a high-level of uniformity is required.
U.S. Pat. No. 4,709,205, issued Nov. 24, 1987 to Baurand et al., which is incorporated herein by reference, shows an attempt to address some of these problems. To eliminate saturation, hysteresis, and other non-linear effects of ferromagnetic cores, they produced a coreless sensor. To eliminate the cost of a toroidally wound core, they arrange four linear coils in the shape of a square to enclose the AC conductor. This arrangement is somewhat sensitive to the position of the AC conductor and to external fields, and this is partly addressed by having a small opening for the conductor which restricts it from moving too far off center. This limits the range of conductor sizes that are suitable for use with this sensor. Also, there is a problematic trade-off between signal strength and the cost of the coils. If the coils have few turns, they can be made relatively inexpensively, however, the signal output is then very weak. The signal output can be greatly increased by increasing the number turns, but this greatly increases the cost because a greater number of turns makes it more difficult to maintain a uniform winding. If the winding is not uniform, then the sensor is more sensitive to the position of the AC conductor, and to external fields and this can lead to increased signal error. Even if the coils are perfectly wound for this geometry, there will be some sensitivity to these sources of error, because four linear coils do not form a true toroid.
U.S. Pat. No. 5,414,400, issued May 9, 1995 to Gris et al., which is incorporated herein by reference, proposes a toroidal coil produced as a printed circuit board. While this greatly reduces the cost, it does suffer from certain difficulties. For example, the resolution of the printed circuit board production process limits the number of turns possible for a given size of sensor. Furthermore, the capture area of each coil is very small, being limited by the radial length of the coil and the thickness of the printed circuit board. As a result, the signal from the sensor is very weak. They have proposed to address this by stacking a number of these printed circuit boards together and combining their signals. However, if a large number of circuit boards are used, this detracts from the cost savings, and still results in a signal that is quite weak. This is because the geometry that they have chosen is quite inefficient in terms of the coil capture area achieved for the amount of printed circuit board required by the coil.
Another problem is that the AC conductor must be strung through the center of the sensor. This means that an existing circuit would have to be disconnected in order to install the sensor. It would of course be possible to modify their printed circuit board design such that the toroid is split in two halves. However, to prevent shorting, it would be necessary to have a gap in the conductor traces where the two halves meet. Such a gap would be a source of non-uniformity and make the sensor sensitive to the position of the AC conductor and to external fields. The effect of the non-uniformity could be reduced by making equivalent gaps evenly spaced around the circumference, but the effect of this would be to further reduce the already low amount of coil capture area.
U.S. Pat. No. 6,271,655, issued Aug. 7, 2001 to Weber et al., which is incorporated herein by reference, presents a current transformer based on a planar coil etched onto a printed circuit board. Said planar coil is in fact a surface coil wherein in this case the surface on which the coil is disposed is planar (or a plane). While they have clearly presented a much more efficient coil geometry, this non-toroidal sensor is particularly sensitive to the position of the AC conductor. This problem is addressed by making the AC conductor a pair of traces on the opposite side of the printed circuit board. Thus, motion of the AC conductor is not possible relative to the sense coil, and it is therefore prevented from being a source of error. While this is not a problem for a device which is assembled into a new product being manufactured, it does make it very inconvenient for use as a general-purpose instrument, or as a sensor which needs to be retrofitted onto an existing conductor. Also, since the AC current must pass through a trace on a printed circuit board, this device is limited to measuring relatively small current levels. Furthermore, because this geometry does not even begin to approach that of a toroid, it can be expected to have a very high level of sensitivity to external fields which cause noise.